The present invention relates to an image sensor used in facsimile machine and image scanner for converting an optical signal into an electric signal.
Conventionally, there is known an image sensor operative such that a photocarrier is stored in a base of a bipolar transistor and an electric siganl signal is taken from an emitter thereof. FIG. 5 shows one example of such a conventional image sensor. A bipolar transistor 1 of NPN type has a collector 2 held at constant potential V.sub.CC. An emitter 3 of the bipolar transistor 1 is connected to a common signal line 5 through a pixel selecting switch 4. A base 6 of the bipolar transistor 1 is held floating. The pixel selecting switches 4 are successively turned on to read out a photocarrier stored in the respective bipolar transisters 1. During the reading operation, the bipolar transistor can output electric charge several tens to hundreds times as much as the stored photocarrier according to the amplification function of the transistor.
In the above noted conventional image sensor, since the base 6 of the bipolar transistor 1 is held floating, some amount of electric charge remains in the base after the previous output operation, thereby affecting the next output operation to generate a storage image phenomenon. Further, since an emitter junction must be forward-biased to effect transistor operation, a part of the photocarrier is used for forward-biasing of the emitter junction. Therefore, as shown in FIG. 4, the output power cannot linearly increase in a lower light exposure range.